(a) A parallel beam of light is incident on a face of a prism with refracting angle $60^{\circ}$. The angle of minimum deviation is found to be $30^{\circ}$. The refractive index of the material of the prism is close to : (A) $1\cdot3$ (B) $1\cdot4$ (C) $1\cdot5$ (D) $1\cdot6$ OR (b) The angle of minimum deviation for a ray of light incident on a thin prism, made of crown glass ($n = 1\cdot52$) is $D_{\text{m}}$. If the prism was made of dense flint glass ($n = 1\cdot62$) instead of crown glass, the angle of minimum deviation will : (A) decrease by $4%$ (B) increase by $4%$ (C) decrease by $19%$ (D) increase by $19%$